First atmospheric measurements of the wind lidar mission Aeolus

September 2018

After a development period of 16 years, the ESA satellite Aeolus was launched into space on a Vega rocket on 22 August 2018 at 21:20 UTC. Named after the Greek God of the Winds, the satellite carries the revolutionary instrument ALADIN (Atmospheric Laser Doppler Lidar Instrument) - the first European lidar and the world's first ever Doppler wind lidar in space. It consists essentially of a pulsed ultraviolet laser, a telescope with a diameter of 1.50 meters and a highly sensitive optical spectrometer. The emitted laser pulses are scattered in the atmosphere from air molecules, aerosols and cloud particles that move with the wind. A small portion returns to the satellite. The wind speed is then derived from the frequency difference between emitted and backscattered pulses, which is caused by the Doppler effect, while the travel time of the pulses contains the altitude information. In this manner, global wind profiles are created from the ground up to 30 kilometers altitude with an accuracy of about 2 m/s.

The Earth Explorer satellite Aeolus was launched onboard a Vega rocket on 22/08/2018 at 21:20 UTC from the European Spaceport Kourou in French Guiana. Copyright: ESA - S. Corvaja

In addition to the testing of the novel technology, the wind data supplied by the instrument will be directly assimilated in the weather forecasts. This closes a large gap in the global observation system, as the network of wind measurements is still very patchy, particularly over the oceans, over the tropics and above ten kilometers altitude. The utilization of wind data in the weather models, as used e.g. by the ECMWF or the German Weather Service DWD, should significantly improve medium-term forecasts. The algorithms and processors needed to derive wind profiles from ALADIN's raw data were developed by a European team of the DLR institutes IPA and IMF, the software company DoRIT as well as several European meteorological services (ECMWF, Météo-France and the Dutch weather service KNMI).

The goal of the Aeolus mission is not only to provide a deeper understanding of the dynamics in the atmosphere. It is also an important contribution to improving weather forecasts. The world's first wind lidar in space provides wind profiles as well as information about aerosols and clouds in the lower 30 km of the atmosphere along the satellite orbit. ESA/ATG medialab.

The technical and scientific functionality of the satellite instrument has already been demonstrated in recent years with a prototype, the ALADIN Airborne Demonstrator (A2D). The instrument, developed by DLR and Airbus, was deployed in several campaigns aboard the research aircraft Falcon to accurately measure, for example, the North Atlantic jet stream. This allowed us to validate the measurement principle, to optimize the operation procedures and to improve the wind retrieval algorithms already before the launch of the satellite. During the three-year satellite mission, which has now begun, the A2D will be used in further upcoming validation campaigns.

To analyze the multitude of data that Aeolus will supply during the mission, the innovative visualization tool VirES for Aeolus was developed by EOX in Vienna in cooperation with DLR and DoRIT. The VirES for Aeolus tool has recently provided a very promising glimpse of the data that Aeolus will deliver over the next few years. Only two weeks after the start of the satellite, first range-resolved backscatter signals were detected. Oliver Reitebuch is delighted with ALADIN's first lidar measurements: "I would not have expected us to be able to analyze the first signals from the atmosphere, which even seem plausible, already two weeks after the satellite was launched and only three days after the laser was switched on. After more than 15 years of intensive involvement in the development of Aeolus, the algorithms for the ALADIN instrument and the experience with the airborne prototype A2D, this is a great success and an example of what can be achieved in European collaboration between ESA, industry and science. This opens a new window for active remote sensing using spaceborne lidar."

The first Aeolus data from 5 September 2018, showing backscattered laser light from molecules (yellow / green) and cloud tops (red-brown). Visualization and analysis of the measurement data is facilitated by the software VirES for Aeolus, which was developed by the company EOX in cooperation with the DLR and DoRIT.